There are some particular items, such as ice cream, juice, or erotic magazines that can stall Koro-sensei and give you more time to assassinate him.
A mission is cleared when you manage to kill Koro-sensei within the time limit.
Assassination Classroom: Grand Siege on Korosensei
Additionally, you can choose to team up with other students or playmates and Koro-sensei can fight back.
There are some particular items, the map for items, traps and weapons. A player must look around you manage to kill Koro-sensei within the time limit.